Polyvinyl alcohol-based (hereinafter, also referred to as “PVA-based”) resins are used as water-soluble resins in a variety of fields such as coating agents, water-soluble binders, emulsification dispersants, and admixtures for civil engineering and construction materials (gypsum and cement). Particularly, when a PVA-based resin in the form of particles is dissolved in water for use, despite that the PVA-based resin particles are highly soluble in water, the surfaces thereof are swollen before the particles are dispersed in water and the particles are thereby agglomerated to form lumps, as a result of which a required amount of the PVA-based resin is not dissolved and partially remains undissolved in some cases. This tendency is prominent especially in PVA-based resin particles having a high low-temperature solubility. Moreover, the agglomerated particles that are left undissolved may be swollen and form large lumps to not only cause a delay in the dissolving operation but also interfere with processes by adhering to a stirring blade and a flow path.
With regard to these problems, various methods of improving dispersibility of PVA-based resin particles in water are conventionally known.
For example, Patent Document 1 discloses a technology for improving dispersibility of a water-soluble polymer in water by bringing water-soluble polymer in the form of fine particles into contact with 0.1 wt % to 10 wt % of hydrophobic fumed silica to coat the hydrophobic fumed silica with water-soluble polymer.
Further, since PVA-based resin particles having a high low-temperature solubility are likely to form lumps, it has been examined to suppress lump formation by using low-solubility PVA-based resin particles; however, there is a problem that such suppression of lump formation leads to a reduction in solubility. In view of this, Patent Literature 2 discloses a technology for obtaining a polyvinyl alcohol-based polymer having a high improvement effect in terms of solubility, which polymer not only can be dispersed without lump formation when added to warm water of 50° C. or higher or hot water but also is completely dissolved at 90° C. to 95° C. to yield an aqueous solution, by producing a polyvinyl alcohol-based polymer having few branches with introduction of a specific catalyst to a polymerization system under specific conditions to improve the crystallinity and by further restricting the saponification conditions.
Moreover, Patent Literature 3 discloses a technology for dissolving water-soluble polymer particles, in which a water-soluble polymer having a high dissolution rate in water is used as a nucleus and is coated with a water-soluble polymer having a low water solubility, into water, warm water or hot water without lump formation.